Interactions Increase Forager Availability and Activity in Harvester Ants

Abstract

Social insect colonies use interactions among workers to regulate collective behavior. Harvester ant foragers interact in a chamber just inside the nest entrance, here called the 'entrance chamber'. Previous studies of the activation of foragers in red harvester ants show that an outgoing forager inside the nest experiences an increase in brief antennal contacts before it leaves the nest to forage. Here we compare the interaction rate experienced by foragers that left the nest and ants that did not. We found that ants in the entrance chamber that leave the nest to forage experienced more interactions than ants that descend to the deeper nest without foraging. Additionally, we found that the availability of foragers in the entrance chamber is associated with the rate of forager return. An increase in the rate of forager return leads to an increase in the rate at which ants descend to the deeper nest, which then stimulates more ants to ascend into the entrance chamber. Thus a higher rate of forager return leads to more available foragers in the entrance chamber. The highest density of interactions occurs near the nest entrance and the entrances of the tunnels from the entrance chamber to the deeper nest. Local interactions with returning foragers regulate both the activation of waiting foragers and the number of foragers available to be activated.

Effect of Interactions between Harvester Ants on Forager Decisions

“…Previous work, supported by the regression fits in Figure 4, suggests that interactions with returning foragers provide evidence to leave the nest to forage: ants that leave the nest to forage tend to interact at a higher rate than ants that return to the deeper nest (Pinter-Wollman et al, 2013; Pless et al, 2015), and the overall rate of outgoing foragers depends on the rate of incoming foragers (Gordon et al, 2008, 2011; Prabhakar et al, 2012; Pinter-Wollman et al, 2013; Pless et al, 2015). Below, we examine how these key results could arise from a decision-making process in which potential foragers sequentially sample interactions with returning foragers and use a stochastic accumulation process to make foraging decisions.…”

“…Previous studies together indicate that interactions with successful foragers returning from trips outside the nest are most relevant for a potential forager’s decision whether to leave the nest to forage, and that ants that leave the nest to forage tend to interact at a higher rate than ants that return to the deeper nest (Schafer et al, 2006; Greene et al, 2013; Pinter-Wollman et al, 2013; Pless et al, 2015). Here, building on previous results, we count only the interactions of potential foragers with foragers returning from trips outside the nest.…”

“…Several studies have demonstrated that the overall rate of outgoing foragers varies positively with the rate of successful returning foragers (Prabhakar et al, 2012; Pinter-Wollman et al, 2013; Pless et al, 2015). To examine how the average rate of returning foragers from outside influences colony foraging patterns, we varied the Poisson input rate in the basic 4-parameter ( γ , k , s 0 , and σ ) model and observed the effect on the fraction of ants that left the nest to forage.…”

“…While previous work shows that foragers use interaction rate inside the nest to decide whether to leave (Gordon et al, 2011; Pinter-Wollman et al, 2013; Pless et al, 2015), it is not known exactly how a forager assesses interaction rate to make its decision. Here we provide a modeling framework to describe how ants use evidence from interactions with returning foragers to decide whether to leave the nest to forage.…”

“…Here we consider foraging decisions at the level of individual ants, asking how each potential forager ant’s rate of interactions predicts the probability it will leave the nest. Potential forager ants come into the nest entrance chamber and choose between two alternative actions (Pless et al, 2015): (1) leave the nest to forage, or (2) return to the deeper nest and not forage. We first present data from field observations and apply a regression model to analyze how an individual ant’s rate of interaction, number of interactions, and time interval over which these interactions occur are correlated with the ant’s foraging decision.…”